Hundreds of marchers descended on the offices of FTSE 100 miner Vedanta Resources in Zambia after explosive comments by its executive chairman about the firm’s vast profits.

The mass protest reignites the row over whether London-listed miners, whose shares are part of the pension portfolios of thousands of Britons, are paying their fair share of tax in poor countries.

The march was sparked by a video showing Vedanta boss Anil Agarwal boasting that the firm has made up to $1billion every year for nearly a decade from its Konkola Copper Mines (KCM) subsidiary.

Video nasty: Protesters in Zambia after Vedanta's chairman boasted about the company's profits

The statement, in a speech posted on video website YouTube, appeared to be at odds with what Vedanta has previously reported about 79 per cent-owned KCM.

The Zambian Revenue Authority has already begun a probe into Vedanta over the comments, amid strained relations between the firm and the governm ent over jobs and tax. But campaigners said Vedanta (up 30p to 1127p) should now be subjected to a full audit, to ensure that it is not funnelling cash out of the country.

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Zambia remains among the world’s poorest nations, despite being Africa’s largest producer of the copper used in electrical wiring and plumbing pipes. Its finance ministry has previously estimated annual losses of $2billion from tax avoidance, with its vast mining industry singled out as the worst culprit.

Tax campaigners said Agarwal’s comments, from a speech in Bangalore, India, cast doubt on Vedanta’s own contribution. In the speech, Agarwal said Vedanta has made ‘between $500million and $1billion’ every year for nine years from KCM.

Vedanta’s last set of accounts show a $15.9million operating loss from its Zambian operations, with a $63.6million profit the year before.

The apparent discrepancy has elicited fury in Zambia, where firms including commodities giant Glencore have previously been accused of massaging figures to minimise tax payments.

Vedanta said Agarwal’s comments had been taken ‘negatively out of context’. Agarwal elaborated on his remarks, saying that the firm had invested nearly $3billion in KCM and taken ‘very little out’.

In a separate statement, Vedanta said it ‘always fully adheres to the highest regulation and disclosure requirements. As such, there should be no doubt over the accuracy of our reporting or the taxes we are paying.’

But campaigners said the response did not explain why the figures in Agarwal’s speech differ so markedly from its reporting.

Murray Worthy, tax expert at ActionAid, said: ‘Vedanta must now disclose a full breakdown of where Agarwal’s figures come from and how they can claim such large earnings while reporting minimal profits. Only then will we know if the Zambian government needs to reclaim unpaid tax urgently needed to pay for healthcare and education.’

Louise Rouse, of responsible investment group ShareAction, which has holdings in all FTSE 100 firms, said the row proves that ‘tax practices of multinational companies are under a public and political spotlight’. She added: ‘Investors need to ensure they are similarly scrutinising tax practices to ensure investee companies don’t suffer “the Starbucks effect”. Shareholders should support calls for greater transparency of revenues earned and taxes paid to enable them to properly assess the risks being run by their companies.’

A spokesman for Christian Aid said the Zambian taxman should extend inquiries to Glencore’s Mopani Copper Mines subsidiary, which insists that tax evasion allegations in 2011 were based on a flawed audit. However, the result of a probe by the European Investment Bank was kept secret, fuelling concerns.